[SimplifyCFG] Don't mistake icmp of and for a tree of comparisons

SimplifyCFG tries to turn complex branch conditions into a switch.
Some of it's logic attempts to reason about bitwise arithmetic produced
by InstCombine.  InstCombine can turn things like (X == 2) || (X == 3)
into (X & 1) == 2 and so SimplifyCFG tries to detect when this occurs so
that it can produce a switch instruction.

However, the legality checking was not sufficient to determine whether
or not this had occured.  Correctly check this case by requiring that
the right-hand side of the comparison be a power of two.

This fixes PR26323.

llvm-svn: 258904

1 files changed, 3 insertions, 2 deletions

diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index b6d87c22e43..57adcecbdfb 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -405,13 +405,14 @@ private:
     ConstantInt *RHSC;
 
     // Pattern match a special case
-    // (x & ~2^x) == y --> x == y || x == y|2^x
+    // (x & ~2^z) == y --> x == y || x == y|2^z
     // This undoes a transformation done by instcombine to fuse 2 compares.
     if (ICI->getPredicate() == (isEQ ? ICmpInst::ICMP_EQ:ICmpInst::ICMP_NE)) {
       if (match(ICI->getOperand(0),
                 m_And(m_Value(RHSVal), m_ConstantInt(RHSC)))) {
         APInt Not = ~RHSC->getValue();
-        if (Not.isPowerOf2()) {
+        if (Not.isPowerOf2() && C->getValue().isPowerOf2() &&
+            Not != C->getValue()) {
           // If we already have a value for the switch, it has to match!
           if(!setValueOnce(RHSVal))
             return false;